Abstract. We present the results of airborne measurements of carbon monoxide (CO) and
aerosol particle number concentration (CN) made during the Balanço
Atmosférico Regional de Carbono na Amazônia (BARCA) program. The
primary goal of BARCA is to address the question of basin-scale sources and
sinks of CO2 and other atmospheric carbon species, a central issue of
the Large-scale Biosphere-Atmosphere (LBA) program. The experiment consisted
of two aircraft campaigns during November–December 2008 (BARCA-A) and
May–June 2009 (BARCA-B), which covered the altitude range from the surface
up to about 4500 m, and spanned most of the Amazon Basin.

Based on meteorological analysis and measurements of the tracer, SF6,
we found that airmasses over the Amazon Basin during the late dry season
(BARCA-A, November 2008) originated predominantly from the Southern
Hemisphere, while during the late wet season (BARCA-B, May 2009) low-level
airmasses were dominated by northern-hemispheric inflow and mid-tropospheric
airmasses were of mixed origin. In BARCA-A we found strong influence of
biomass burning emissions on the composition of the atmosphere over much of
the Amazon Basin, with CO enhancements up to 300 ppb and CN concentrations
approaching 10 000 cm−3; the highest values were in the southern part
of the Basin at altitudes of 1–3 km. The ΔCN/ΔCO ratios were
diagnostic for biomass burning emissions, and were lower in aged than in
fresh smoke. Fresh emissions indicated CO/CO2 and CN/CO emission ratios
in good agreement with previous work, but our results also highlight the
need to consider the residual smoldering combustion that takes place after
the active flaming phase of deforestation fires.

During the late wet season, in contrast, there was little evidence for a
significant presence of biomass smoke. Low CN concentrations
(300–500 cm−3) prevailed basinwide, and CO mixing ratios were enhanced by only
~10 ppb above the mixing line between Northern and Southern Hemisphere
air. There was no detectable trend in CO with distance from the coast, but
there was a small enhancement of CO in the boundary layer suggesting diffuse
biogenic sources from photochemical degradation of biogenic volatile organic
compounds or direct biological emission.

Simulations of CO distributions during BARCA-A using a range of models
yielded general agreement in spatial distribution and confirm the important
contribution from biomass burning emissions, but the models evidence some
systematic quantitative differences compared to observed CO concentrations.
These mismatches appear to be related to problems with the accuracy of the
global background fields, the role of vertical transport and biomass smoke
injection height, the choice of model resolution, and reliability and
temporal resolution of the emissions data base.